Wastewater treatment system

ABSTRACT

The wastewater treatment system of the present invention is adapted to remove COD and nitrogenous compounds from wastewater. The system includes a carbon-removing anaerobic fluidized bed reactor and a nitrogen-removing fluidized bed reactor. The carbon-removing anaerobic fluidized bed reactor is mainly adapted to transfer most of the COD in the wastewater into methane through hydrolysis, acedogenesis and methanogenosis reactions. The nitrogen-removing fluidized bed reactor is adapted to transfer ammonium nitrogen and residual COD in the wastewater into nitrogen gas through partial nitrification, anammox and denitrification reactions.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to wastewater treatment, and more particularly to a wastewater treatment system that uses microorganisms to treat COD and ammonium nitrogen in water.

2. Description of Related Art

In the field of biological wastewater treatment, the traditional nitrification-denitrification method was once predominant until a recently introduced technology based on anammox bacteria have been regarded better in terms of energy efficiency and increasingly adopted. During anaerobic ammonium oxidation, ammonium nitrogen and nitrite nitrogen act as an electron donor and an electron acceptor, respectively, and are then transferred into nitrogen gas and nitrate nitrogen.

It is reported that anammox bacteria is more suitable for wastewater having high ammonium nitrogen concentration (with ammonium nitrogen concentration greater than 500 mg N/L). One reason for this is that since anammox bacteria grow slowly, if the supply of ammonium nitrogen is not abundant, the startup time of the relevant biological reactor may be significantly increased and, in worse cases, the establishment of a reaction system based on anammox bacteria may totally fail. It is known that municipal wastewater has relatively low ammonium nitrogen concentration, which is typically 20-85 mg N/L, and for this reason anammox bacteria are considered ineffective in treating municipal wastewater. Besides, domestic wastewater usually has its COD higher than ammonium nitrogen concentration. When COD is extremely high, a reactor based on anammox bacteria may fail to remove COD effectively.

SUMMARY OF THE INVENTION

In view of this, the primary objective of the present invention is to provide a wastewater treatment system effective in removing at least some COD and nitrogenous compounds from wastewater.

For achieving the foregoing and other objectives, the disclosed wastewater treatment system for at least partially removing COD, ammonium nitrogen and other nitrogenous compounds from wastewater comprises a carbon-removing anaerobic fluidized bed reactor and a nitrogen-removing fluidized bed reactor. The carbon-removing anaerobic fluidized bed reactor includes a first cylinder, a plurality of first granular carriers, a first sedimentation tank, a first fluidized means, first microorganisms and extracellular enzymes. The first cylinder defines therein a first fluidized chamber. The first cylinder has a first upper opening and a first lower opening. The first upper and lower openings are both communicated with the first fluidized chamber. The first fluidized chamber is locally filled by the first granular carriers. The first sedimentation tank has a first bottom opening and a first outfall locatioanlly higher than the first bottom opening. The first bottom opening is communicated with the first upper opening. The first fluidized means serves to guide wastewater into the first fluidized chamber from the first lower opening and serves to suspend the first granular carriers in first fluidized chamber. A part of the COD performs hydrolysis reaction with said extracellular enzymes to decompose organic compounds of the COD into at least one of amino acids, carbohydrates and fatty acids. At least one part of said first microorganisms is attached to the first granular carriers. Said first microorganisms include acidogenic bacteria and methanogens, wherein the acidogenic bacteria perform acedogenesis reaction to transfer at least one of said amino acids, carbohydrates and fatty acids into fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide, and the methanogens perform methanogenesis reaction to transfer the fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide into methane and carbon dioxide. Said first fluidized chamber has an oxidation-reduction potential therein smaller than −400 mv. The first outfall serves to drain the wastewater from the carbon-removing anaerobic fluidized bed reactor. The nitrogen-removing fluidized bed reactor includes a second cylinder, a plurality of second granular carriers, a second sedimentation tank, a second fluidized means and a second microorganism. The second cylinder defines therein a second fluidized chamber. The second cylinder has a second upper opening and a second lower opening. The second upper and lower openings are both communicated with the second fluidized chamber. The second lower opening serves to introduce the wastewater drained from the first outfall. The second fluidized chamber is locally filled by the second granular carriers. The second sedimentation tank has a second bottom opening and a second outfall locationally higher than the second bottom opening. The second bottom opening is communicated with the second upper opening. The second fluidized means serves to guide the wastewater into the second fluidized chamber through the second lower opening and serves to suspend the second granular carriers in the second fluidized chamber. At least one part of said second microorganisms is attached to the second granular carriers. Said second microorganisms include nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria. The nitrifying bacteria perform partial nitrification reaction to oxidize ammonium nitrogen into nitrite nitrogen. The anammox bacteria perform anammox reaction to oxidize ammonium nitrogen and nitrite nitrogen into nitrogen gas and nitrate nitrogen. The heterotrophic denitrifying bacteria perform denitrification reaction to transfer nitrate nitrogen and at least a part of the residual COD into nitrogen gas.

For achieving the foregoing and other objectives, the present invention further provides a wastewater treatment system for at least partially removing COD and nitrogenous compounds that include ammonium nitrogen from wastewater. The wastewater treatment system includes a carbon-removing anaerobic fluidized bed reactor, an anaerobic fluidized membrane reactor and a nitrogen-removing fluidized bed reactor. The carbon-removing anaerobic fluidized bed reactor includes a first cylinder, a plurality of first granular carriers, a first sedimentation tank, a first fluidized means, first microorganisms and extracellular enzymes. The first cylinder defines therein a first fluidized chamber. The first cylinder has a first upper opening and a first lower opening. The first upper and lower openings are both communicated with the first fluidized chamber. The first fluidized chamber is locally filled by the first granular carriers. The first sedimentation tank has a first bottom opening and a first outfall locationally higher than first bottom opening. The first bottom opening is communicated with first upper opening. The first fluidized means serves to guide the wastewater from the first lower opening into the first fluidized chamber, and serves to suspend the first granular carriers in the first fluidized chamber. A part of the COD performs hydrolysis reaction with said extracellular enzymes to decompose organic compounds of the COD into at least one of amino acids, carbohydrates and fatty acids. At least one part of said first microorganisms is attached to the first granular carriers. Said first microorganisms include acidogenic bacteria and methanogens, wherein the acidogenic bacteria perform acedogenesis reaction to transfer at least one of said amino acids, carbohydrates and fatty acids into fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide, and the methanogens perform methanogenesis reaction to transfer the fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide into methane and carbon dioxide. Said first fluidized chamber has an oxidation-reduction potential therein smaller than −400 mv. The first outfall serves to drain the wastewater from the carbon-removing anaerobic fluidized bed reactor. The anaerobic fluidized membrane reactor includes a third cylinder, a plurality of third granular carriers, a third sedimentation tank, a third fluidized means and at least one tubular membrane. The third cylinder defines therein a third fluidized chamber. The third cylinder has a third upper opening and a third lower opening. The third upper and lower openings are both communicated with the third fluidized chamber. The third lower opening serves to introduce the wastewater drained from the first outfall. The third fluidized chamber is locally filled by the third granular carriers. The third sedimentation tank has a third outfall. The third sedimentation tank is located above the third cylinder and is communicated with the third fluidized chamber through the tubular membrane. The tubular membrane extends from the third sedimentation tank into the third fluidized chamber and is defined by a porous wall. The third fluidized means serves to guide the wastewater into the third fluidized chamber through the third lower opening and serves to suspend the third granular carriers in the third fluidized chamber. The third outfall serves to drain the wastewater from the anaerobic fluidized membrane reactor. The nitrogen-removing fluidized bed reactor includes a second cylinder, a plurality of second granular carriers, a second sedimentation tank, a second fluidized means and second microorganisms. The second cylinder defines therein a second fluidized chamber. The second cylinder has a second upper opening and a second lower opening. The second upper and lower openings are both communicated with the second fluidized chamber. The second lower opening serves to introduce the wastewater drain from the third outfall. The second fluidized chamber is locally filled by the second granular carriers. The second sedimentation tank has a second bottom opening and a second outfall locationally higher than second bottom opening. The second bottom opening is communicated with second upper opening. The second fluidized means serves to guide the wastewater into the second fluidized chamber through the second lower opening and serves to suspend the second granular carriers in the second fluidized chamber. At least one part of said second microorganisms is attached to the second granular carriers. Said second microorganisms include nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria. The nitrifying bacteria perform partial nitrification reaction to oxidize ammonium nitrogen into nitrite nitrogen. The anammox bacteria perform anammox reaction to oxidize ammonium nitrogen and nitrite nitrogen into nitrogen gas and nitrate nitrogen. The heterotrophic denitrifying bacteria perform denitrification reaction to transfer nitrate nitrogen and at least a part of the residual COD into nitrogen gas.

The disclosed wastewater treatment system advantageously requires significantly reduced startup time as compared to any known wastewater treatment, and provides effective nitrogen removal even for wastewater having low ammonium nitrogen concentration. The disclosed wastewater treatment system is capable of transferring most COD into methane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a wastewater treatment system according to one embodiment of the present invention;

FIG. 2 according to one embodiment of the present invention shows variations of ammonium nitrogen concentration and ammonium nitrogen removal rate over time;

FIG. 3 according to one embodiment of the present invention shows variations of total target nitrogen concentration and total target nitrogen removal rate over time.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a wastewater treatment system according to one embodiment of the present invention is configured to at least partially remove COD and nitrogenous compounds including ammonium nitrogen from wastewater. The wastewater treatment system has a carbon-removing anaerobic fluidized bed reactor 10, an anaerobic fluidized membrane reactor 20, and a nitrogen-removing fluidized bed reactor 30. The disclosed wastewater treatment system has its front half mainly serving to remove carbon-containing compounds from water and has its rear half mainly serving to remove nitrogenous compounds from water.

The carbon-removing anaerobic fluidized bed reactor 10 includes a first cylinder 11, a plurality of first granular carriers 12, a first sedimentation tank 13, a first fluidized means, first microorganisms and extracellular enzymes.

The first cylinder 11 defines therein a first fluidized chamber 111. The first cylinder 11 has a first upper opening 112 and a first lower opening 113. The first upper and lower openings 112, 113 are both communicated with the first fluidized chamber 111. The first upper opening 112 is provided at the top of the first cylinder 11. The first lower opening 113 is provided at the bottom of the cylinder 11 for introducing wastewater.

The first fluidized chamber 111 is locally filled by the first granular carriers 12. In the present embodiment, the first granular carriers 12 are beads of natural zeolite but are not limited thereto.

The first sedimentation tank 13 has a first bottom opening 131 and a first outfall 132 locationally higher than first bottom opening 131. The first sedimentation tank 13 is provided at the top of the first cylinder 11. The first bottom opening 131 is communicated with first upper opening 112. The first sedimentation tank 13 is atop provided with a methane vent 133.

The first fluidized means serves to guide the wastewater into the first fluidized chamber 111 through the first lower opening 113 and serves to suspend the first granular carriers 12 in the first fluidized chamber 111. In one application, the first granular carriers 12 do not enter the first sedimentation tank 13 because of the first fluidized means. The first fluidized means includes any equipment that can generate upward flow in the first fluidized chamber 111, such as water pumps like a magnetic pump 141 and/or a peristaltic pump 142. The water pumps may be used in any number as long as the generated upward flow is speedy enough to suspend the first granular carriers 12.

At least one part of the first microorganisms is attached to the first granular carriers 12. Said first microorganisms include acidogenic bacteria and methanogens, such as Methanosaeta spp. Said extracellular enzymes refer to enzymes synthesized intracellularly and then secreted outside to act extracellularly and these extracellular enzymes are supportive to hydrolysis reaction. A part of the COD in the wastewater perform hydrolysis reaction with the extracellular enzymes, so that the organic compounds forming the COD are decomposed into at least one of amino acids, carbohydrates and fatty acids. The acidogenic bacteria perform acedogenesis reaction to transfer at least one of said amino acids, carbohydrates and fatty acids into fatty acids having 4 or fewer carbons on their backbone chains (such as acetic acid, propanoic acid, butyric acid), hydrogen gas and carbon dioxide. The methanogens perform methanogenesis reaction to transfer the fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide into methane and carbon dioxide, also known as biogas. The first fluidized chamber 111 maintains an anaerobic environment therein without oxygen supply, and has its oxidation-reduction potential smaller than −400 mv. The nitrogenous compounds in the COD have been also transferred into ammonium nitrogen in the precious process. The wastewater processed through hydrolysis reaction, acedogenesis reaction and methanogenesis reaction is then drained from the first outfall 132. At least one part of the generated methane and carbon dioxide exhausts through the methane vent 133 and is collected. The total reaction in the first fluidized chamber 111 may be expressed by the following reaction formula:

$\left. {{C_{a}H_{b}O_{c}N_{d}} + {\left( \frac{{4a} - b - {2c} - {3d}}{4} \right)H_{2}O}}\rightarrow{{\frac{{4a} - b - {2c} - {3d}}{8}{CH}_{4}} + {\frac{{4a} - b - {2c} - {3d}}{8}{CO}_{2}} + {d{NH}}_{3}} \right.$

For acclimating the first microorganisms in the first fluidized chamber 111, activesludge carrying the first microorganisms may be input into the first fluidized chamber 111. At least one part of the first microorganisms is then attached to the first granular carriers 12 and grows. In one example, the activesludge input was acquired from the anaerobic digester in Linkou Wastewater Treatment Plant (Taiwan) and the amount input was 500 ml. The mixed liquor suspended solid (MLSS) concentration was 22.5 g/L, and the mixed liquor volatile suspended solid (MLVSS) concentration was 5.5 g/L.

The anaerobic fluidized membrane reactor 20 includes a third cylinder 21, a plurality of third granular carriers 22, a third sedimentation tank 23, a third fluidized means and one or more tubular membranes 24.

The third cylinder 21 defines therein a third fluidized chamber 211 and has a third upper opening 212 and a third lower opening 213. The third upper and lower openings 212, 213 are both communicated with the third fluidized chamber 211. The third upper and lower openings 212, 213 are located at two ends of the third cylinder 21, respectively. The third lower opening 213 serves to introduce the wastewater drained from the first outfall 132. The third fluidized chamber 211 also maintains therein an anaerobic environment without particular aeration. In one possible embodiment, the third fluidized chamber 211 also has its oxidation-reduction potential smaller than −400 mv.

The third fluidized chamber 211 is locally filled by the third granular carriers 22. In the present embodiment, the third granular carriers 22 are beads of natural zeolite, but are not limited thereto.

The third sedimentation tank 23 has a third outfall 231. The third sedimentation tank 23 is provided at the top of the third cylinder 21. The third sedimentation tank 23 is communicated with the third fluidized chamber 211 through the tubular membrane 24. The tubular membrane 24 extends from the lower part of the third sedimentation tank 23 into the third fluidized chamber 211. The tubular membrane 24 is defined by a porous wall. The third sedimentation tank 23 and the third fluidized chamber 211 are not communicated directly and there may be a partition arranged between the third sedimentation tank 23 and the third fluidized chamber 211. In the present embodiment, the tubular membrane is a hollow fiber film having an outer diameter of 1.2 mm, an inner pore size smaller than 0.1 μm and a total membrane surface area of 0.08 m².

The third fluidized means serves to guide the wastewater into the third fluidized chamber 211 through the third lower opening 213 and serves to suspend the third granular carriers 22 in the third fluidized chamber 211. In one application, the third granular carriers 22 do not enter the third sedimentation tank 23 because of the third fluidized means. The third fluidized means includes any equipment that can generate upward flow in the third fluidized chamber 211, such as water pumps like a magnetic pump 251 and/or a peristaltic pump 252. The water pumps may be used in any number as long as the generated upward flow is speedy enough to suspend the third granular carriers 22. As the third granular carriers 22 are lifted by the upward flow, they naturally scrub the surface tubular membrane 24, thereby eliminating the need of cleaning the tubular membrane with chemicals. The anaerobic fluidized membrane reactor 20 maintains therein an anaerobic environment without oxygen supply, and mainly serves to remove the suspended solids form the wastewater. The processed wastewater is then drained through the third outfall 231.

For verifying how effectively the carbon-removing anaerobic fluidized bed reactor 10 and the anaerobic fluidized membrane reactor 20 remove COD and suspended solids, some tests have been conducted with the conditions described below. In one test, domestic wastewater was continuously introduced into the carbon-removing anaerobic fluidized bed reactor 10 through the first lower opening 113. The domestic wastewater was obtained from the wastewater treatment facility of National Chiao Tung University, Taiwan. The organic loading rate (OLR) of the carbon-removing anaerobic fluidized bed reactor 10 was controlled at 1.75-4.7 Kg/m³/d. The hydraulic retention time (HRT) for the carbon-removing anaerobic fluidized bed reactor 10 was 1 hour. The membrane flux of the anaerobic fluidized membrane reactor 20 was controlled at 8.33-12.5 LMH. The hydraulic retention time for the anaerobic fluidized membrane reactor 20 was 2-3 hours. The test lasted for 111 days, and the results are listed in Table I below. In Table I, AFBR represents the carbon-removing anaerobic fluidized bed reactor; AFMBR represents the anaerobic fluidized membrane reactor; TSS represents total suspended solids; VSS represents volatile suspended solids and TKN represents total Kjeldahl nitrogen. In the table, except for pH, all items for Influent and Effluent are recoded in mg/L.

TABLE I Sample Effluent Removal Rate (%) Item No. Influent AFBR AFMBR AFBR AFBR + AFMBR pH 30 7.15 ± 0.21 7.01 ± 0.08 7.19 ± 0.1  COD 26 130 ± 38  42 ± 11 20 ± 5  66 ± 12 84 ± 5 TSS 12 58 ± 31 12 ± 10 2 ± 3 74 ± 28 96 ± 7 VSS 12 44 ± 18 9 ± 8 1 ± 1 74 ± 5  97 ± 5 TKN 4 61 ± 22 48 ± 7  34 ± 7  Ammonium 11 42 ± 15 51 ± 15 47 ± 16 Nitrogen Nitrate 11 3 ± 4 2 ± 4 3 ± 4 Nitrogen Nitrite 11 0 0 0 Nitrogen

As demonstrated by the results, the carbon-removing anaerobic fluidized bed reactor 10 and the anaerobic fluidized membrane reactor 20 achieved a total COD removal rate of 70-90% and a total suspended solid removal rate up to 96%. As the carbon-removing anaerobic fluidized bed reactor 10 when solely used provided desired removal, the anaerobic fluidized membrane reactor 20 may be omitted in some possible embodiments. During the process, some microorganisms and extracellular enzymes in the carbon-removing anaerobic fluidized bed reactor 10 might flow into the anaerobic fluidized membrane reactor 20 with wastewater. Since the anaerobic fluidized membrane reactor 20 also maintained therein an anaerobic environment, some of the COD was transferred into methane and carbon dioxide in the anaerobic fluidized membrane reactor 20. The carbon-removing anaerobic fluidized bed reactor 10 and the anaerobic fluidized membrane reactor 20 achieved a specific methane production of 0.13 L CH₄/g COD_(removed), equal to energy of 0.0024 kWh/m³.

On the other hand, the nitrogen-removing fluidized bed reactor 30 includes a second cylinder 31, a plurality of second granular carriers 32, a second sedimentation tank 33, an aerating apparatus 34, a second fluidized means and second microorganisms.

The second cylinder 31 defines therein a second fluidized chamber 311. The second cylinder 31 has a second upper opening 312 and a second lower opening 313. The second upper and lower openings 312, 313 are both communicated with the second fluidized chamber 311. The second lower opening 313 serves to introduce the wastewater drained by the third outfall 231.

The second fluidized chamber 311 is locally filled by the second granular carriers 32. In the present embodiment, the second granular carriers 32 are bioballs, which are plastic beads with grooves on their surfaces (AQUARIUM CO., LTD, Taiwan), but are not limited thereto.

The second sedimentation tank 33 has a second bottom opening 331 and a second outfall 332 locationally higher than the second bottom opening 331. The second bottom opening 331 is communicated with second upper opening 313. The second sedimentation tank 33 is atop provided with a vent 333 for exhausting nitrogen gas generated during the treatment.

The aerating apparatus 34 has an aerating end 341 that extends from the second sedimentation tank 33 into the second cylinder 31, for maintaining the dissolved oxygen concentration in the second fluidized chamber 311 at 0.1-0.5 mg/L.

The second fluidized means serves to guide the wastewater into the second fluidized chamber 311 through the second lower opening 313 and serves to suspend the second granular carriers 32 in the second fluidized chamber 311. In one application, the second granular carriers 32 do not enter the second sedimentation tank 33 because of the second fluidized means. The second fluidized means includes any equipment that can generate upward flow in the second fluidized chamber 311, such as water pumps like a magnetic pump 351 and/or a peristaltic pump 352. The water pumps may be used in any number as long as the generated upward flow is speedy enough to suspend the second granular carriers 32.

At least one part of the second microorganisms is attached to the second granular carriers 32. The second microorganisms include nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria. The nitrifying bacteria perform partial nitrification reaction to oxidize ammonium nitrogen in the wastewater into nitrite nitrogen. The anammox bacteria perform anammox reaction to transfer ammonium nitrogen and nitrite nitrogen in the wastewater into nitrogen gas and nitrate nitrogen. The heterotrophic denitrifying bacteria perform denitrification reaction to transfer nitrate nitrogen and at least some residual COD in the wastewater into ammonium nitrogen.

For acclimating the microorganisms in the second fluidized chamber 311, activesludge carrying the second microorganisms may be input to the second fluidized chamber 311. As the acclimation proceeds, at least some microorganisms are attached to the second granular carriers 32 and grow. In one example, activesludge acquired from a landfill-leachate treatment facility in Taipei, Taiwan was used. The activesludge was input during the startup stage of the nitrogen-removing fluidized bed reactor 30. In the startup stage of the nitrogen-removing fluidized bed reactor 30, the activesludge was first input to the fluidized chamber of the nitrogen-removing fluidized bed reactor 30. The nitrogen-removing fluidized bed reactor 30 was operated in the startup stage with the conditions listed in Table II below. In the example, no sludge discharge was performed during the startup stage.

TABLE II Item Condition Temperature Room Temperature Flow 2 L/min Sludge Retention Time Indefinite Duration Sludge Concentration MLSS: 4725 mg/L MLVSS: 3510 mg/L

Then, wastewater was introduced into the second fluidized chamber 311 through the second lower opening 313 of the nitrogen-removing fluidized bed reactor 30. The wastewater used is secondary settling pool wastewater coming from the secondary settling pool of Taoyuan Wastewater Treatment Plant (Taiwan). The quality of the wastewater is shown in Table III. Therein, TTN refers to total target nitrogen, and total target nitrogen concentration is the sum of the concentration values of ammonium nitrogen, nitrite nitrogen, and nitrate nitrogen.

TABLE III Concentration Concentration Parameter (mg/L) Parameter (mg/L) Ammonium Nitrogen 26 ± 4 COD 25 ± 16 Nitrite Nitrogen  0 ± 0 TSS 7 ± 8 Nitrate Nitrogen  2 ± 1 VSS 4 ± 3 TTN 28 ± 5

After the nitrogen-removing fluidized bed reactor 30 is started, the second granular carriers 32 are carried by the wastewater and suspend in the second fluidized chamber 311. The second microorganisms attached to and growing on the second granular carriers 32 undergo partial nitrification reaction, anammox reaction and denitrification reaction at the same time in the second fluidized chamber 311. The wastewater flows into the second fluidized chamber 311 through the second lower opening 313, and then passes through the second upper opening 312, the second bottom opening 331 and the second outfall 332 of the second sedimentation tank 33. The wastewater has a hydraulic retention time in the second fluidized chamber 311 of 12-24 hours. In one example, the hydraulic retention time for the nitrogen-removing fluidized bed reactor 30 was 24 hours for Days 1-28, and 18 hours for Days 29-63.

The test results are provided in Table IV below and in FIGS. 2 and 3. As evidenced, the total average ammonium nitrogen removal rate is 98.3%. The removal rate was as high as 93.5% on the first day of reaction and stayed steadily above 70% since Day 1, and above 80% since Day 9, with an average at 99.7%. To discuss the ammonium nitrogen removal rate with consideration of different lengths of hydraulic retention time, the 24 h hydraulic retention time (for Days 1-28) led to an average ammonium nitrogen removal rate of 96.1% and the 18 h hydraulic retention time (for Days 29-63) led to an average ammonium nitrogen removal rate of 99.7%. On the other hand, the total average TTN removal rate is 91.3%. The TTN removal rate achieved 75.8% on Day 1, and stayed steadily above 80% since Day 9, with an average at 95.6%. The 24 h hydraulic retention time (for Days 1-28) led to an average TTN removal rate of 87.2%, and the 18 h hydraulic retention time (for Days 29-63) led to an average TTN removal rate of 96.3%.

TABLE IV Hydraulic Retention Time (HRT) Parameter 18 Hours 24 Hours TTN 1 ± 1 mg/L 2 ± 0 mg/L Ammonium Nitrogen 0 ± 0 mg/L 0 ± 0 mg/L Nitrite Nitrogen 0 ± 0 mg/L 0 ± 0 mg/L Nitrate Nitrogen 1 ± 1 mg/L 2 ± 0 mg/L COD 13 ± 5 mg/L  17 ± 3 mg/L  TSS 2 ± 5 mg/L 2 ± 1 mg/L VSS 1 ± 1 mg/L 2 ± 1 mg/L

It is evident that the nitrogen-removing fluidized bed reactor 30 of the present invention provides effective nitrogen removal even for wastewater having low ammonium nitrogen concentration. Additionally, the present invention significantly reduces the startup time as compared to the known treatment methods or to the use of other reactors. For example, the system and method disclosed in Taiwan Patent No. 201429884 involve the use of a sequencing batch reactor. According to one example provided in the prior patent, synthesized wastewater was introduced in the startup stage and microorganisms such as nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria were used to remove nitrogen from the synthesized wastewater. The synthesized wastewater had its ammonium nitrogen concentration of 400-600 mg/L. With these settings, it takes about 90 days to complete the startup stage and reach stable TTN removal rate above 80%, and the ammonium nitrogen removal rate was not steady and close to 100% until Day 330. Daverey et al. (Achlesh Daverey, Nien-Tzu Hung, Kasturi Dutta, Jih-Gaw LinChen. 2013. Ambient temperature SNAD process treating anaerobic digester liquor of swine wastewater. Bioresource Technology 141: 191-198) also used a sequencing batch reactor to treat swine wastewater. During the startup stage, the ammonium nitrogen removal rate did not steady until Day 60-70, after which the rate stayed at 80%, and the TTN removal rate did not reach 75% until Day 75, and finally reached 80% after 480 days from the beginning. Keluskar et al. (Radhika Keluskar, Anuradha Nerurkar, Anjana Desai. 2013. Development of a simultaneous partial nitrification, anaerobic ammonia oxidation and denitrification (SNAD) bench scale process for removal of ammonia from effluent of a fertilizer industry. Bioresource Technology 130: 390-397) instead used a cylindrical reactor for fertilizer industry wastewater. During the startup stage, it took almost 30 days for the ammonium nitrogen removal rate to reach 80%.

Generally speaking, when the efficiency of nitrogen removal is stable above 80%, the system is deemed as started. With this definition, it is found that the disclosed nitrogen-removing fluidized bed reactor helps significantly reduce the required startup time. It is also found that the nitrogen-removing fluidized bed reactor is applicable to wastewater that contains low concentration of ammonium nitrogen, such as municipal wastewater that usually has its ammonium nitrogen concentration of 20-85 mg/L. The conclusion overturns the traditional knowledge saying that anammox bacteria are ineffective in treating domestic wastewater.

It is to be noted that acclimation of the first microorganisms and acclimation of the second microorganisms may be conducted separately. In this case, the carbon-removing anaerobic fluidized bed reactor 10 and the nitrogen-removing fluidized bed reactor 30 are connected only after the both microorganisms have been acclimated. The anaerobic fluidized membrane reactor 20 is aimed at removing suspended solids from wastewater. Thus, in one possible embodiment, the anaerobic fluidized membrane reactor 20 may be omitted, in which case the wastewater drained by the first outfall 132 is introduced into the second fluidized chamber 311 through the second lower opening 313. In another possible embodiment, an additional fluidized membrane reactor may be provided downstream the nitrogen-removing fluidized bed reactor for removing suspended solids from wastewater. While the carbon-removing anaerobic fluidized bed reactor 10, the anaerobic fluidized membrane reactor 20 and the nitrogen-removing fluidized bed reactor 30 in the present embodiment all include additional sedimentation tanks 15, 25, 35 for facilitating sediment of suspended solids, these sedimentation tanks 15, 25, 35 may be omitted in other embodiments.

To sum up, the disclosed wastewater treatment system is effective in reducing COD and nitrogenous compounds in wastewater and is applicable to domestic wastewater that contains low concentration of target matters. The process can generate biogas like methane, which can be further converted into energy. Also, suspended solid concentration in the effluent can be significantly reduced to ensure environmentally conformable water emission. Hence, the present invention does possess great potential to become the next-generation technology for biological wastewater treatment. 

What is claimed is:
 1. A wastewater treatment system, for at least partially removing COD and nitrogenous compounds from wastewater, the nitrogenous compounds including ammonium nitrogen, the wastewater treatment system comprising: a carbon-removing anaerobic fluidized bed reactor, including a first cylinder, a plurality of first granular carriers, a first sedimentation tank, a first fluidized means, first microorganisms and extracellular enzymes; the first cylinder defining therein a first fluidized chamber, the first cylinder having a first upper opening and a first lower opening, the first upper and lower openings both being communicated with the first fluidized chamber; the first fluidized chamber being locally filled by the first granular carriers; the first sedimentation tank having a first bottom opening and a first outfall locationally higher than the first bottom opening, the first bottom opening being communicated with the first upper opening; the first fluidized means being for guiding the wastewater into the first fluidized chamber through the first lower opening and for suspending the first granular carriers in first fluidized chamber; a part of the COD performing hydrolysis reaction with said extracellular enzymes to decompose organic compounds of the COD into at least one of amino acids, carbohydrates and fatty acids, at least one part of said first microorganisms being attached to the first granular carriers, said first microorganisms including acidogenic bacteria and methanogens, wherein the acidogenic bacteria perform acedogenesis reaction to transfer said at least one of amino acids, carbohydrates and fatty acids into fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide, and the methanogens perform methanogenesis reaction to transfer the fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide into methane and carbon dioxide, said first fluidized chamber having an oxidation-reduction potential therein smaller than −400 mv; the first outfall allowing the wastewater to be drained from the carbon-removing anaerobic fluidized bed reactor; and a nitrogen-removing fluidized bed reactor, including a second cylinder, a plurality of second granular carriers, a second sedimentation tank, a second fluidized means and second microorganisms; the second cylinder defining therein a second fluidized chamber, the second cylinder having a second upper opening and a second lower opening, the second upper and lower openings both being communicated with the second fluidized chamber, the second lower opening serving to introduce the wastewater that has been processed in the carbon-removing anaerobic fluidized bed reactor; the second fluidized chamber being locally filled by the second granular carriers; the second sedimentation tank having a second bottom opening and a second outfall locationally higher than the second bottom opening, the second bottom opening being communicated with the second upper opening; the second fluidized means being for guiding the wastewater into the second fluidized chamber through the second lower opening, and for suspending the second granular carriers in the second fluidized chamber; at least one part of said second microorganisms being attached to the second granular carriers, said second microorganisms including nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria, the nitrifying bacteria performing partial nitrification reaction to oxidize ammonium nitrogen into nitrite nitrogen, the anammox bacteria performing anammox reaction to transfer ammonium nitrogen and nitrite nitrogen into nitrogen gas and nitrate nitrogen, the heterotrophic denitrifying bacteria performing denitrification reaction to transfer nitrate nitrogen and at least a part of the residual COD into nitrogen gas.
 2. A wastewater treatment system, for at least partially removing COD and nitrogenous compounds from wastewater, nitrogenous compounds includes ammonium nitrogen, the wastewater treatment system comprising: a carbon-removing anaerobic fluidized bed reactor, including a first cylinder, a plurality of first granular carriers, a first sedimentation tank, a first fluidized means, first microorganisms and extracellular enzymes; the first cylinder defining therein a first fluidized chamber, the first cylinder having a first upper opening and a first lower opening, the first upper and lower openings both being communicated with the first fluidized chamber; the first fluidized chamber being locally filled by the first granular carriers; the first sedimentation tank having a first bottom opening and a first outfall locationally higher than the first bottom opening, the first bottom opening being communicated with the first upper opening; the first fluidized means being for guiding the wastewater into the first fluidized chamber through the first lower opening and for suspending the first granular carriers in first fluidized chamber; a part of the COD performing hydrolysis reaction with said extracellular enzymes to decompose organic compounds forming the COD into at least one of amino acids, carbohydrates and fatty acids, at least one part of said first microorganisms being attached to the first granular carriers, said first microorganisms including acidogenic bacteria and methanogens, wherein the acidogenic bacteria perform acedogenesis reaction to transfer said at least one of amino acids, carbohydrates and fatty acids into fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide, and the methanogens perform methanogenesis reaction to transfer the fatty acids having 4 or fewer carbons on their backbone chains, hydrogen gas and carbon dioxide into methane and carbon dioxide, said first fluidized chamber having an oxidation-reduction potential therein smaller than −400 mv; the first outfall serving to drain the wastewater that has been processed through hydrolysis reaction, acedogenesis reaction and methanogenesis reaction; an anaerobic fluidized membrane reactor, including a third cylinder, a plurality of third granular carriers, a third sedimentation tank, a third fluidized means and at least one tubular membrane, the third cylinder defining therein a third fluidized chamber, the third cylinder having a third upper opening and a third lower opening, the third upper and lower openings both being communicated with the third fluidized chamber, the third lower opening serving to introduce the wastewater that has been processed in the carbon-removing anaerobic fluidized bed reactor; the third fluidized chamber being locally filled by the third granular carriers; the third sedimentation tank having a third outfall, the third sedimentation tank is provided atop the third cylinder, the third sedimentation tank being communicated with the third fluidized chamber through the tubular membrane, the tubular membrane extending form the third sedimentation tank into the third fluidized chamber, the tubular membrane being defined by a porous wall, the third fluidized means being for guiding the wastewater into the third fluidized chamber through the third lower opening and for suspending the third granular carriers in the third fluidized chamber; the third outfall serving to drain the wastewater from the anaerobic fluidized membrane reactor; and a nitrogen-removing fluidized bed reactor, including a second cylinder, a plurality of second granular carriers, a second sedimentation tank, a second fluidized means and second microorganisms; the second cylinder defining therein a second fluidized chamber, the second cylinder having a second upper opening and a second lower opening, the second upper and lower openings both being communicated with the second fluidized chamber, said second lower opening serving to introduce the wastewater that has been processed in the anaerobic fluidized membrane reactor; the second fluidized chamber being locally filled by the second granular carriers; the second sedimentation tank having a second bottom opening and a second outfall locationally higher than the second bottom opening, the second bottom opening being communicated with the second upper opening; the second fluidized means being for guiding the wastewater into the second fluidized chamber through the second lower opening, and for suspending the second granular carriers in the second fluidized chamber; at least one part of said second microorganisms being attached to the second granular carriers, said second microorganisms including nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria, the nitrifying bacteria performing partial nitrification reaction to oxidize ammonium nitrogen into nitrite nitrogen, the anammox bacteria performing anammox reaction to transfer ammonium nitrogen and nitrite nitrogen into nitrogen gas and nitrate nitrogen, the heterotrophic denitrifying bacteria performing denitrification reaction to transfer nitrate nitrogen and at least a part of the residual COD into nitrogen gas.
 3. The wastewater treatment system of claim 1, wherein a dissolved oxygen concentration in the second fluidized chamber is 0.1-0.5 mg/L.
 4. The wastewater treatment system of claim 2, wherein a dissolved oxygen concentration in the second fluidized chamber is 0.1-0.5 mg/L.
 5. The wastewater treatment system of claim 1, wherein the first sedimentation tank is atop provided with a methane vent.
 6. The wastewater treatment system of claim 2, wherein the first sedimentation tank is atop provided with a methane vent. 